The present invention relates to a process for fractional distillation under vacuum, and in particular to such a process wherein liquid to be distilled is passed in a relatively thin film through a depressurized vessel. Part of the liquid film is caused to vaporize, the formed vapor is collected on a condensing surface for condensing the vapor to form liquid distillate whereafter the formed liquid distillate and the non-vaporized part of the liquid film, i.e. the residue, are separately recovered from the vessel.
The above type of distillation, which is also called molecular distillation, is particularly suitable for distilling heavy liquids with extremely high boiling points. It is normally conducted at pressures in the range of a few microns Hg absolute, which means a high vacuum compared to distillation in conventional vacuum units operating at a few mm Hg absolute at least. Molecular distillation might therefore advantageously be applied for further distilling residues from conventional high vacuum distillation units. It is known to carry out short path distillation under vacuum by providing a heated vaporizing solid surface and a condensing surface adjacent thereto, and supplying to the vaporizing surface the liquid to be distilled. The condensing surface employed may be a solid surface, suitably cooled, or may comprise films of a condensing liquid. When using such a heated solid surface for vaporization, care should be taken to prevent coke formation on the surface due to decomposition of the liquid on the relatively hot vaporizing surface.
To overcome the problem of coke formation it has also been proposed to heat the liquid to a vaporizing temperature prior to forming the liquid into a vaporizing film flowing along a guiding surface. Special measures should however be taken to insure that the liquid is spread out in a thin film over the evaporating surface, to enable all the volatile components in the liquid to vaporize and leave the remaining liquid. Although coke formation on the non-heated guiding surfaces is considerably reduced compared to the application of heated guiding surfaces, there remains a problem of fouling of the guiding surfaces. This occurs especially when very heavy liquids are to be distilled, the construction of the distillation unit should be such that the unit can be easily and rapidly dismantled for cleaning or replacement of the guiding surfaces.
Although small installations with restricted capacities may be constructed to provide a good accessibility to the internals, problems may occur with large units intended for high liquid throughputs. In the latter case care should be taken that the internals can be easily replaced and that the accessibility of such a unit is not accompanied with an impairment of the evacuability of the unit.